Intramolecular interconversion

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

Attempts have been made to observe1084 the assumed interconversion of 625 to 626 in fluorenyl cations. Such intramolecular interconversion through the capped pyramidal ion 627 was not observed [Eq. (3.150)]. MINDO/3 calculations1084 on the isomeric structures of cyclopentadienyl, indenyl, and fluorenyl cations indicated strongly decreasing relative stabilities of pyramidal forms due to bezannulation. 

Intramolecular Interconversions in Cyclosilazane Chemistry A Joint Experimental-Theoretical Study... 

Interestingly, Peters (1988) notices that the methylene protons are enantiotopic for the light lanthanides (R = Ce-Dy) and become diastereotopic for R = Ho-Yb which implies a dynamic intramolecular interconversion between the two helical enantiomers P- R(Ll-2H)3]3 M-[7 (L1-2H)3]3 occurring at a moderate rate on the NMR time scale. The... 

The dimetallic platinum complex [ (C6F5)2Pt 2(K2-pzTp)] has been characterized by NMR which shows the occurrence of a very fast intramolecular interconversion between the conformational isomers arising from the boat-like M(N-N)2B ring inversion (Fig. 2.131).528... 

High activation barriers for such reactions give rise to a particularly interesting situation in the case of nondegenerate systems, namely, a thermodynamically unstable tautomer can be isolated under conditions that would exclude all but the intramolecular interconversion channels. Such conditions occur in gas-phase reactions under deep vacuum, they are particularly effective in interstellar space. 

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